Try connecting the drive to a different computer, to see if your computer is the problem.
If necessary, open the external case, and separate the disk-drive from the USB-to-disk-drive adapter.
Connect the disk-drive as a "slave" disk-drive in some computer, to see if the disk-drive itself still can be accessed.
If not, and you have "important" data on the disk-drive, search the Internet for a professional "Data Recovery Service" -- you ship your disk-drive to them, pay them a lot of money, and they repair the disk-drive, just long-enough to copy your files to a brand-new disk-drive.
Answers & Comments
Try connecting the drive to a different computer, to see if your computer is the problem.
If necessary, open the external case, and separate the disk-drive from the USB-to-disk-drive adapter.
Connect the disk-drive as a "slave" disk-drive in some computer, to see if the disk-drive itself still can be accessed.
If not, and you have "important" data on the disk-drive, search the Internet for a professional "Data Recovery Service" -- you ship your disk-drive to them, pay them a lot of money, and they repair the disk-drive, just long-enough to copy your files to a brand-new disk-drive.